1. Field of the Invention
The invention relates generally to methods for increasing absorption of compounds by an animal and particularly to the use of hormones such as 5-methoxy-N-acetyltryptamine to increase absorption of peptides, peptidomimetics, and other gastrointestinal transport protein substrates by an animal.
2. Description of the Related Art
Peptide transporter proteins are integral membrane proteins that mediate the cellular uptake of di- and tripeptides and a variety of peptidomimetics and other compounds. They are found in bacteria, yeast, plants, invertebrates, and vertebrates. In vertebrates, two transporter proteins, Peptide transporter 1 and 2, designated PepT1 and PepT2 respectively, are expressed predominantly in brush border membranes of small intestine (PepT1), kidney (PepT1 and PepT2), and lung (PepT2). The PepT transporters are proton-coupled transporters capable of transporting numerous dipeptides and tripeptides, as well as a large spectrum of therapeutic drugs like β-lactam antibiotics, selected angiotensin-converting enzyme (ACE) inhibitors, and peptidase inhibitors. It is believed that substrate flux is coupled to proton movement down an electrochemical proton gradient with the membrane potential as the main driving force for the translocation.
Mammalian oligopeptide transporters are part of the PTR2 family of membrane transporters. They are characterized by two signature motifs that are conserved in all known family members. The first conserved motif is a region that begins at the end of the second putative transmembrane domain, including the following first cytoplasmic loop as well as the third transmembrane domain. The second conserved motif corresponds to the core region of the fifth transmembrane region. Besides the mammalian PepT1 and PepT2 transporter proteins, the PTR2 family includes the yeast peptide transporter PTR2, DtpT from Lactococcus lactis, and numerous “orphan” transporters having unknown functions. Most orphan transporters are found in prokaryotes such as Escherichia coli. 
In mammals, PepT1 plays an important role in the absorption of proteins, including small oligopeptides. PepT1 is expressed primarily in the brush-border membranes of intestinal epithelial cells where it mediates the transport of oligopeptides, such as those found in digests, from the gut lumen into the cells. In addition, because of its relatively broad substrate specificity, PepT1 can accept various pharmacologically-active compounds, including β-lactam antibiotics, and serve as an absorptive pathway for these compounds. Functional studies have shown that, in addition to utilizing the H+ electrochemical gradient as a driving force, PepT1 exhibits pH-dependence, and is reported to be a high-velocity, low-affinity transporter. PepT1 is also reportedly an inducible transporter. Inducers include substrates, substrate analogs, and various hormones.
Physiological stresses, including intestinal illness, physical stress, surgery, injury, and/or mental stress can negatively influence intestinal health and function, directly or indirectly affecting amino acid absorption. For example, it has been shown that rats with Type 1 diabetes (i.e., naturally low or absent insulin production) had a reduced PepT1 activity compared to healthy rats.
Attempts have been made to improve amino acid absorption. For example pre-digesting dietary protein and adding the hydrolyzed dietary protein to foods, such as critical care foods has been used, however, such approaches primarily increase the pool of amino acids or peptides available for absorption, and do not necessarily enhance the absorption process. Some specific small peptides can apparently stimulate peptide absorption. Methods of feeding specific peptides, particularly glycylsarcosine or β-alanyl-L-histidine (carnosine), to increase peptide absorption in a canine cell line were disclosed in U.S. Pat. No. 6,803,186, but in vivo, the use of these peptides was not successful in stimulating peptide absorption compared to the standard diet alone. It has been indicated that a dipeptide, alanyl-glutamine, alone was not sufficient to stimulate peptide absorption activity; however, when combined with growth hormone (GH), peptide absorption was maintained when a human cell line was subjected to oxidative stress by hydrogen peroxide. Peroxide-induced oxidative stress reduced peptide absorption in the absence of peptide and GH. Some scientific literature indicates that certain active biological peptides or hormones can stimulate small peptide absorption, through directly stimulating the activity of PepT1. Insulin, epidermal growth factor, leptin, interferon-gamma, and thyroid hormone have all shown some stimulating effect. Conflicting data has been reported regarding any stimulatory effect of growth hormone.
With the exception of thyroid hormone, the stimulatory peptide hormones are species-specific proteins. Thus, application or use of these in practice would require, for example, synthesis by chemical or recombinant gene techniques for each species in which the hormones were to be used. Further, it appears that these hormones have been administered by subcutaneous injection, which is not desirable in actual application.
Other efforts have been made to specifically regulate the transport activity of PepT1 for various purposes. For example, U.S. Patent Application US20030170748A1 discloses nucleic acids encoding a canine PepT1. The publication also evaluated the absorption of certain peptide-bound forms of leucine and tryptophan, as well as the ability of various small peptides to inhibit the transport of a model substrate, glycylsarcosine (GlySar). Methods are provided for determining whether a particular peptide is likely to have a beneficial nutritional property to an animal, and for stimulating PepT1 activity in cells by contacting the cells with a PepT1 substrate. Similarly, U.S. Patent Application US20060210569A1 discloses methods of inhibiting PepT1, thereby inhibiting cell-growth.
There remains a need, therefore, for methods of increasing absorption or uptake in an animal of one or more specified or unspecified peptides, peptidomimetics and other gastrointestinal transport protein substrates, particularly through transporter proteins such as the PepT1 transporter.